This invention relates to a group frame synchronization system for the parallel transmission of a plurality of digital signals such as a plurality of time-division multiplexed telephone signals and, more particularily, to a group frame synchronization system for establishing the group frame synchronization between those digital signals, which are transmitted in parallel by the PSK(Phase-Shift-Keying)-modulation of a carrier wave of extremely high frequency region such as the millimeter band.
In the conventional group frame synchronization system of this kind, a plurality of mutually synchronized digital signals are respectively converted at the transmitting end to signals of a higher bit rate. Then group frame synchronization signals are inserted into the vacant time slots obtained as a result of the bit-rate conversion. After the synchronization pattern insertion, those digital signals are usually transmitted in parallel by the PSK-modulation of a carrier wave of millimeter or quasi-millimeter band. In the case of a four-phase PSK system, two signals can be transmitted in parallel according to such method. Similarly, in the case of an eight-phase PSK system, three signals can be transmitted in parallel. Beside the PSK modulation, a multi-level amplitude modulation system or a combined phase-amplitude modulation system may also be used.
At the receiving end, the inserted group frame synchronization signals are extracted from the transmitted digital signals, thereby to separate those transmitted digital signals from one another. The separated digital signals are then subjected to a bit rate reduction so that the original bit rate may be restored. Based on the respective individual frame synchronization signals included therein, the digital signals are respectively channel-separated to reproduce analog signals such as telephone signals.
In the conventional group frame synchronization of this type, a framing signal detector and a frame synchronization are needed for each one of the digital signals to be transmitted. Furthermore, the use of the mutually different frame synchronization code patterns needed for such conventional systems has generally necessitated the provision, at the receiving end, of the frame synchronization signal generators equal in number to the digital signals to be transmitted in parallel.
On the other hand, in the parallel transmission of a plurality of digital signals by the use of a carrier wave of millimeter or quasi-millimeter band, the so-called scrambling is applied at the transmitting end to each digital signal at an interval corresponding to the group frame synchronization signal. At the receiving end, the transmitted digital signal is descrambled to maintain the transmission quality of the time-division multiplexed channels. Further, in some cases, all the digital signals are transmitted with monitoring information superposed thereon or inserted therein in common to monitor the transmission line. To achieve the scrambling and the monitoring information insertion economically, the frame construction must be identical for all the digital signals so that they may be controlled in common. However, the conventional frame synchronization system does not permit this easily because the synchronization pattern is different from one digital signal to another.